Internet Engineering Task Force S. Floyd
Internet-Draft M. Allman
Intended status: Best Current Practice ICIR / ICSI
Expires: November 2007 May 2007
Specifying New Congestion Control Algorithmsdraft-ietf-tsvwg-cc-alt-03.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The IETF's standard congestion control schemes have been widely
shown to be inadequate for various environments (e.g., high-speed
networks). Recent research has yielded many alternate congestion
control schemes that significantly differ from the IETF's congestion
control principles. Using these new congestion control schemes in
the global Internet has possible ramifications to both the traffic
using the new congestion control and to traffic using the currently
standardized congestion control. Therefore, the IETF must proceed
with caution when dealing with alternate congestion control
proposals. The goal of this document is to provide guidance for
considering alternate congestion control algorithms within the IETF.
TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:
Changes from draft-ietf-tsvwg-cc-alt-01.txt:
* Removed references from abstract.
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* Added a note that we are focused on documents produced within the
IETF (i.e., these are not guidelines that the IRTF or the RFC
Editor would necessarily have to follow).
* Added a list of 'difficult environments' the IETF has thought
about in the past (even while admitting that an exhaustive list of
'difficult environments' is impossible to produce).
* Removed section 5 (conclusions). Some felt that it was redundant
and not needed.
* Made a few of the references normative.
* Various small wording tweaks.
Changes from draft-ietf-tsvwg-cc-alt-01.txt:
* Very minor wording tweaks gathered during WGLC.
Changes from draft-ietf-tsvwg-cc-alt-00.txt:
* Added text to the introduction to clarify the relationship of this
document and RFC 2914. In addition, added a requirement (0) in
section 3 that says new congestion control schemes that
significantly diverge from the principles in RFC 2914 must explain
this divergence.
Changes from draft-floyd-tsvwg-cc-alt-00.txt:
* Changed the name to draft-ietf-tsvwg-cc-alt-00.txt.
* Added a sentence about robustness with various
queueing algorithms in the routers, especially both RED
and DropTail. Suggestion from Jitendra Padhye.
* Added a sentence about robustness with the routers,
middleboxes, and such deployed in the current Internet.
Concern taken from a talk by Henry Sanders.
* Add a section about minimum requirements necessary for
approval for deployment in the global Internet.
Suggestion by Jitendra Padhye.
* Added more examples to guideline 3 about difficult environments,
and added that TCP performance in difficult environments is
still an active research topic. Suggestion from Doug Leith.
* Added citations to examples of discussions of these issues
in Experimental RFCs 3649 and 4782.
* Added examples of high speed TCP proposals. Suggestion
from Bob Braden.
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* Changed the fairness bullets to better reflect that new congestion
controllers are expected to assess the impact to standard
congestion controlled flows---without commenting on how that
assessment should be done. From discussions with bob Briscoe.
* Made numerous editing changes suggested by Gorry Fairhurst.
Changes from draft-floyd-cc-alt-00.txt:
* Changed the name to draft-floyd-tsvwg-cc-alt-00.txt.
* Added a bullet about incremental deployment. Feedback from
Colin Perkins
* Clarified the fairness section; this section is not saying
that strict TCP-friendliness is a requirement.
* Clarified that as an alternative to Full Backoff, a flow
could stop sending when the packet drop rate is above a
certain threshold.
* Clarified that the Full Backoff bullet does not require
that different flows with different round-trip times
use the same criteria about when they should back off
to one packet per round-trip time or less.
* Added a paragraph about Informational RFCs.
* Added a bullet about response to transient events, including
routing events or moving from a private to a shared network.
END OF NOTES TO BE DELETED.
1. Introduction
This document provides guidelines for the IETF to use when
evaluating suggested congestion control algorithms that
significantly differ from the general congestion control principles
outlined in [RFC2914]. The guidance is intended to be useful to
authors proposing alternate congestion control and for the IETF
community when evaluating whether a proposal is appropriate for
publication in the RFC series.
The guidelines in this document are intended to be consistent with
the congestion control principles from [RFC2914] of preventing
congestion collapse, considering fairness, and optimizing the flow's
own performance in terms of throughput, delay, and loss. [RFC2914]
also discusses the goal of avoiding a congestion control `arms race'
among competing transport protocols.
This document does not give hard-and-fast requirements for an
appropriate congestion control scheme. Rather, the document
provides a set of criteria that should be considered and weighed by
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the IETF in the context of each proposal. The high-order criteria
for any new proposal is that a serious scientific study of the pros
and cons of the proposal needs to have been done such that the IETF
has a well rounded set of information to consider.
After initial studies, we encourage authors to write a specification
of their proposals for publication in the RFC series to allow others
to concretely understand and investigate the wealth of proposals in
this space.
2. Document Status
Following the lead of HighSpeed TCP [RFC3649], alternate congestion
control algorithms are expected to be published as "Experimental"
RFCs until such time that the community better understands the
solution space. Traditionally, the meaning of "Experimental" status
has varied in its use and interpretation. As part of this document
we define two classes of congestion control proposals that can be
published with the "Experimental" status. The first class includes
algorithms that are judged to be safe to deploy for best-effort
traffic in the global Internet and further investigated in that
environment. The second class includes algorithms that, while
promising, are not deemed safe enough for widespread deployment as
best-effort traffic on the Internet, but are being specified to
facilitate investigations in simulation, testbeds, or controlled
environments. The second class can also include algorithms where
the IETF does not yet have sufficient understanding to decide if the
algorithm is or is not safe for deployment on the Internet.
Each alternate congestion control algorithm published is required to
include a statement in the abstract indicating whether or not the
proposal is considered safe for use on the Internet. Each alternate
congestion control algorithm published is also required to include a
statement in the abstract describing environments where the protocol
is not recommended for deployment. There may be environments where
the protocol is deemed *safe* for use, but still is not
*recommended* for use because it does not perform well for the user.
As examples of such statements, [RFC3649] specifying HighSpeed TCP
includes a statement in the abstract stating that the proposal is
Experimental, but may be deployed in the current Internet. In
contrast, the Quick-Start document [RFC4782] includes a paragraph in
the abstract stating the the mechanism is only being proposed for
controlled environments. The abstract specifies environments where
the Quick-Start request could give false positives (and therefore
would be unsafe to deploy). The abstract also specifies
environments where packets containing the Quick-Start request could
be dropped in the network; in such an environment, Quick-Start would
not be unsafe to deploy, but deployment would still not be
recommended because it could cause unnecessary delays for the
connections attempting to use Quick-Start.
For authors of alternate congestion control schemes who are not
ready to bring their congestion control mechanisms to the IETF for
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standardization (either as Experimental or as Proposed Standard),
one possibility would be to submit an internet-draft that documents
the alternate congestion control mechanism for the benefit of the
IETF and IRTF communities. This is particularly encouraged in order
to get algorithm specifications widely disseminated to facilitate
further research. Such an internet-draft could be submitted to be
considered as an Informational RFC, as a first step in the process
towards standardization. Such a document would also be expected to
carry an explicit warning against using the scheme in the global
Internet.
Note: we are not changing RFC publication process for non-IETF
produced documents (e.g., those from the IRTF or independent
RFC-Editor submissions). However, we would hope the guidelines in
this document inform the IESG as they consider whether to add a note
to such documents.
3. Guidelines
As noted above, authors are expected to do a well-rounded evaluation
of the pros and cons of proposals brought to the IETF. The
following are guidelines to help authors and the IETF community.
Concerns that fall outside the scope of these guidelines are
certainly possible; these guidelines should not be considered as an
all-encompassing check-list.
(0) Differences with Congestion Control Principles [RFC2914]
Proposed congestion control mechanisms should include a clear
explanation of the deviations from [RFC2914].
(1) Impact on Standard TCP, SCTP [RFC2960], and DCCP [RFC4340].
Proposed congestion control mechanisms should be evaluated when
competing with standard IETF congestion control
[RFC2581,RFC2960,RFC4340]. Alternate congestion controllers
that have a significantly negative impact on traffic using
standard congestion control may be suspect and this aspect
should be part of the community's decision making with regards
to the suitability of the alternate congestion control
mechanism.
We note that this bullet is not a requirement for strict
TCP-friendliness as a prerequisite for an alternate congestion
control mechanism to advance to Experimental. As an example,
HighSpeed TCP is a congestion control mechanism that is
Experimental, but that is not TCP-friendly in all environments.
We also note that this guideline does not constrain the fairness
offered for non-best-effort traffic.
As an example from an Experimental RFC, fairness with standard
TCP is discussed in Sections 4 and 6 of [RFC3649] (HighSpeed
TCP) and using spare capacity is discussed in Sections 6, 11.1,
and 12 of [RFC3649].
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(2) Difficult Environments.
The proposed algorithms should be assessed in difficult
environments such as paths containing wireless links.
Characteristics of wireless environments are discussed in
[RFC3819] and in Section 16 of [Tools]. Other difficult
environments can include those with multipath routing within a
connection. We note that there is still much to be desired in
terms of the performance of TCP in some of these difficult
environments. For congestion control mechanisms with explicit
feedback from routers, difficult environments can include paths
with non-IP queues at layer-two, IP tunnels, and the like. A
minimum goal for experimental mechanisms proposed for widespread
deployment in the Internet should be that they do not perform
significantly worse than TCP in these environments.
While it is impossible to enumerate all possible "difficult
environments", we note that the IETF has previously grappled
with paths with long delays [RFC2488], high delay bandwidth
products [RFC3649], high packet corruption rates [RFC3155],
packet reordering [RFC4653] and significantly slow links
[RFC3150]. Aspects of alternate congestion control that impact
networks with these characteristics should be detailed.
As an example from an Experimental RFC, performance in difficult
environments is discussed in Sections 6, 9.2, and 10.2 of
[RFC4782] (Quick-Start).
(3) Investigating a Range of Environments.
Similar to the last criteria, proposed alternate congestion
controllers should be assessed in a range of environments. For
instance, proposals should be investigated across a range of
bandwidths, round-trip times, levels of traffic on the reverse
path, and levels of statistical multiplexing at the congested
link. Similarly, proposals should be investigated for robust
performance with different queueing mechanisms in the routers,
especially Random Early Detection (RED) [FJ03] and Drop-Tail.
This evaluation is often not included in the internet-draft
itself, but in related papers cited in the draft.
A particularly important aspect of evaluating a proposal for
standardization is in understanding where the algorithm breaks
down. Therefore, particular attention should be paid to
characterizing the areas where the proposed mechanism does not
perform well.
As an example from an Experimental RFC, performance in a range
of environments is discussed in Section 12 of [RFC3649]
(HighSpeed TCP) and Section 9.7 of [RFC4782] (Quick-Start).
(4) Protection Against Congestion Collapse.
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The alternate congestion control mechanism should either stop
sending when the packet drop rate exceeds some threshold
[RFC3714], or should include some notion of "full backoff". For
"full backoff", at some point the algorithm would reduce the
sending rate to one packet per round-trip time and then
exponentially backoff the time between single packet
transmissions if congestion persists. Exactly when either "full
backoff" or a pause in sending comes into play will be
algorithm-specific. However, as discussed in [RFC2914], this
requirement is crucial to protect the network in times of
extreme congestion.
If "full backoff" is used, this bullet does not require that the
full backoff mechanism must be identical to that of TCP
[RFC2988]. As an example, this bullet does not preclude full
backoff mechanisms that would give flows with different
round-trip times comparable bandwidth during backoff.
(5) Fairness within the Alternate Congestion Control Algorithm.
In environments with multiple competing flows all using the same
alternate congestion control algorithm, the proposal should
explore how bandwidth is shared among the competing flows.
(6) Performance with Misbehaving Nodes and Outside Attackers.
The proposal should explore how the alternate congestion control
mechanism performs with misbehaving senders, receivers, or
routers. In addition, the proposal should explore how the
alternate congestion control mechanism performs with outside
attackers. This can be particularly important for congestion
control mechanisms that involve explicit feedback from routers
along the path.
As an example from an Experimental RFC, performance with
misbehaving nodes and outside attackers is discussed in Sections
9.4, 9.5, and 9.6 of [RFC4782] (Quick-Start). This includes
discussion of misbehaving senders and receivers; collusion
between misbehaving routers; misbehaving middleboxes; and the
potential use of Quick-Start to attack routers or to tie up
available Quick-Start bandwidth.
(7) Responses to Sudden or Transient Events.
The proposal should consider how the alternate congestion
control mechanism would perform in the presence of transient
events such as sudden congestion, a routing change, or a
mobility event. Routing changes, link disconnections,
intermittent link connectivity, and mobility are discussed in
more detail in Section 17 of [Tools].
As an example from an Experimental RFC, response to transient
events is discussed in Section 9.2 of [RFC4782] (Quick-Start).
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(8) Incremental Deployment.
The proposal should discuss whether the alternate congestion
control mechanism allows for incremental deployment in the
targeted environment. For a mechanism targeted for deployment
in the current Internet, it would be helpful for the proposal to
discuss what is known (if anything) about the correct operation
of the mechanism with some of the equipment installed in the
current Internet, e.g., routers, transparent proxies, WAN
optimizers, intrusion detection systems, home routers, and the
like.
As a similar concern, if the alternate congestion control
mechanism is intended only for specific environments, the
proposal should consider how this intention is to be carried
out. For example, if a proposed congestion control scheme is
deemed suitable for deployment in controlled environments but
unsafe for widespread deployment in the Internet, is it
sufficient just to have a sentence in the Abstract of the
document stating this, or are some additional mechanisms needed
as well?
As an example from an Experimental RFC, deployment issues are
discussed in Sections 10.3 and 10.4 of [RFC4782] (Quick-Start).
4. Minimum Requirements
This section suggests minimum requirements for a document to be
approved as Experimental with approval for widespread deployment in
the global Internet. We note that this is not a binding document
with fixed and unchanging requirements, but simply a document
targeted for approval as Best Current Practice.
The minimum requirements for approval for widespread deployment in
the global Internet include the following guidelines (1) on
assessing the impact on standard congestion control, (3) on
investigation of the proposed mechanism in a range of environments,
guideline (4) on protection against congestion collapse and
guideline (8), discussing whether the mechanism allows for
incremental deployment.
For other guidelines, i.e., (2), (5), (6), and (7), evidence that
the proposed mechanism has significantly more problems than those of
TCP should be a cause for concern in approval for widespread
deployment in the global Internet.
6. Security Considerations
This document does not represent a change to any aspect of the
TCP/IP protocol suite and therefore does not directly impact
Internet security. The implementation of various facets of the
Internet's current congestion control algorithms do have security
implications (e.g., as outlined in [RFC2581]). Alternate congestion
control schemes should be mindful of such pitfalls, as well, and
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[RFC3649] S. Floyd, HighSpeed TCP for Large Congestion Windows,
RFC 3649, September 2003.
[RFC3714] S. Floyd and J. Kempf, IAB Concerns Regarding Congestion
Control for Voice Traffic in the Internet, RFC 3714, March 2004.
[RFC3819] P. Karn, C. Bormann, G. Fairhurst, D. Grossman, R. Ludwig,
J. Mahdavi, G. Montenegro, J. Touch, and L. Wood, Advice for Internet
Subnetwork Designers, RFC 3819, July 2004
[RFC4653] Sumitha Bhandarkar, A. L. Narasimha Reddy, Mark Allman,
Ethan Blanton, Improving the Robustness of TCP to Non-Congestion
Events, RFC 4653, August 2006.
[RFC4782] S. Floyd, M. Allman, A. Jain, and P. Sarolahti,
Quick-Start for TCP and IP. RFC 4782, Experimental, January
2007.
[Tools] S. Floyd and E. Kohler, Tools for the Evaluation of
Simulation and Testbed Scenarios, Internet-draft
draft-irtf-tmrg-tools-03.txt, work in progress, December 2006.
Authors' Addresses
Sally Floyd
ICIR (ICSI Center for Internet Research)
1947 Center Street, Suite 600
Berkeley, CA 94704-1198
Phone: +1 (510) 666-2989
Email: floyd at icir.org
URL: http://www.icir.org/floyd/
Mark Allman
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704-1198
Phone: (440) 235-1792
Email: mallman at icir.org
URL: http://www.icir.org/mallman/
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